This invention relates to ophthalmological apparatus, and more particularly to suction cups designed to be applied to the sclera of the eye to modify, for diagnostic purposes, the intraocular pressure.
The alteration of the intraocular pressure in order to study various physiological processes in the human eye is finding increasing usefulness in research and diagnosis. Two different procedures, for different diagnostic purposes, are in common use. In the first of these, which is useful in glaucoma diagnosis, the drainage vessels which normally allow the outflow of the aqueous humor are occluded, and the subsequent pressure rise in the eye over a predetermined time interval is observed in order to determine the rate of formation of the aqueous humor. After removal of the occlusion, the rate of pressure decay in the eye can be monitored to obtain information about the aqueous humor outflow facility. Additionally, the independent measurement of the steady-state intraocular pressure, the rate of aqueous humor formation, and the outflow facility permits the calculation of venous pressure opposing the outflow of aqueous humor. Apparatus useful for carrying out this procedure typically consists of a suction cup dimensioned to fit over and clear of the cornea and provided with a flange designed to contact the perilimbal area. Partial evacuation of the suction cup results in the flange exerting a positive pressure on the perilimbal area, thereby occluding the underlying drainage vessels. An example of such a cup, and apparatus for use therewith, is disclosed in U.S. Pat. No. 3,308,810.
In the second procedure, useful in the detection of carotid artery occlusive disease, a suction cup is applied above the sclera of the patient's eye clear of the area of the cornea. Partial evacuation of the cup results in the physical deformation of the eye which in turn raises the intraocular pressure. Observation of the pulse of the ophthalmic artery as a function of varying intraocular pressure permits the evaluation of the ophthalmic arterial pressure in conscious human subjects. Prior art procedure typically employs apparatus such as that disclosed in U.S. Pat. No. 3,308,810, together with an observation of ocular pulse through, for instance, observations of the fundus, or employs similar suction cups in an apparatus incorporating pneumatic sensing of the applied vacuum, as in U.S. Pat. No. 3,911,903.
Practical considerations in the design of the perilimbal-type cup result in a more or less thin-walled bell-shaped cup having a broad external flexible flange, this shape providing corneal clearance and also insuring that the flange of the cup, by conforming to the shape of the eye, seals the cup to the perilimbal region, part of which is normally covered by the eyelids, with a minimal amount of patient discomfort. The flexible flange and thin-walled cup further allow a reasonably high external pressure to be locally applied to the perilimbal region with minimal negative pressure inside the cup.
Although commonly used for the second procedure, perilimbal cups, because of their shape and size, suffer from a number of disadvantages. For example, the force necessary to distort the eye to raise the intraocular pressure to that of the ophthalmic artery may result in too high an external pressure being placed on the conjunctiva by such a cup, with damage possibly occuring as a result. Further, in applying a suction cup above the sclera of the eye clear of the area of the cornea, the cup is applied to one, typically the temporal, side of the eye, while the patient is instructed to look in the opposite direction. It will be appreciated that such a location of a perilimbal cup, because of its size, results in an even greater displacement of the eyelids, with a substantially greater portion of the flange disposed under the lid and the lid partially retracted by the bell of the cup. Not only does this increase the amount of patient discomfort, but experimentally such a situation should be avoided, as the resultant forces exerted on the cup by the eyelids, being uncontrolled, are a potential source of error. Additionally, for some procedures, it is desirable that the patient fixate straight ahead during measurements; the perilimbal type cups again because of their size require the patient to gaze nasally, thereby preventing forward fixation. This also may place a distrubing stress on the external rectus. Further, the size of the perilimbal cup results in a cup of substantial mass which, by applying a torque on the eye, may indirectly lead to muscular forces of unknown magnitude being placed on the eye in an attempt by the musculature to overcome the torque. Finally, the bell-shaped cup is difficult to handle, in that its abruptly changing diameter offers essentially a choice, for a grip, of either a small diameter stem remote from, or a large diameter bell near to, the eye.
A reduction in the size of the cup is clearly desirable. However, it will be recongized that any reduction in the inside diameter of the cup results in the vacuum producing the distortional force being deployed over a smaller area, which in turn requires that a larger vacuum be employed to produce a given intraocular pressure. Further, it will be recognized that any reduction in flange area results in a greater force per unit area being exerted on the eye. Both can lead to eye damage. At too great a negative pressure differential, it is possible to rupture the capillaries underlying the center of the cup. Too small a contact surface may lead to cutting the conjunctiva with the edge of the cup when distorting forces are applied.